The objective of this proposal is to explore the principal wear mechanisms of experimental dental composites. Eight experimental light-cured composites which contain either bisGMA/TEGDMA or UEDMA/-TEGDMA as matrices, 60 vol% of quartz or barium filler particles, and two different silanization procedures will be prepared, analyzed, and tested under in vivo and in vitro conditions. After an initial characterization of the two resin systems, the degree of conversion of the materials will be determined. In addition, qualitative and quantitative analyses of silanization effectiveness will be performed. These studies will be followed by in vitro evaluations of compressive strength, flexural strength, toughness and wear of the composite systems. The in vitro wear evaluation will be conducted using a wear testing machine (deGee et a/., 1986) and a standard metallographic polishing machine (Jorgensen, 1980). Mechanical properties and wear rate will be determined for three aging conditions: 1) dry storage in a desiccator (37 degrees C), 2) in distilled water (37 degrees C), and 3) in artificial saliva (37 degrees C) for periods up to three years. Filler leaching rates and diffusion coefficients of water molecules from distilled water and artificial saliva in these materials will also be determined. After ensuring that the eight experimental composites have fulfilled the requirements of ADA Specification No. 27, in vivo wear resistance of the composites will be evaluated. Two in vivo investigations will be conducted using 64 patients at the University of Florida and 64 patients at the University of Leuven, Belgium. One American dentist and one Belgian dentist will each treat 32 patients in each country. Class I restorations will be placed in maxillary and mandibular first molars. The material loss due to wear will be determined using three different evaluation techniques after 6, 12, 24, and 36 months. With these clinical evaluations, it will be possible to determine: 1) whether differences in diet among different population groups result in significant differences in wear rate, 2) whether restorations made by different dentists are likely to result in differences in clinical wear performance, and 3) whether differences in wear evaluation methods for the clinical specimens result in significantly different wear results. It will also be determined whether strong correlations exist between any of the investigated properties (including in vitro wear) and the clinical wear rate. In addition, experimental materials containing 60 vol% of filler with mean particle diameters of approximately 0.2, 1, 5 and 10 mu-m will be investigated regarding their in vitro wear resistance. The objective of this part of the study is to determine the influence of filler spacing on wear rate.